1. Field of the Invention
The present invention generally relates to an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine, that includes a fixer, and a fixing method, and more particularly, to an electromagnetic induction heating fixer, an image forming apparatus including the same, and a fixing method using the same.
2. Discussion of the Background Art
In general, an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of those functions, forms an electrostatic latent image on an image carrier, develops the latent image with developer such as toner, and transfers the developed image from the image carrier onto a sheet of recording media, such as paper, overhead projector (OHP) film, and the like, after which, the developed image (toner image) is fixed on the sheet.
A fixer is a mechanism that typically includes a rotary fixing member such as a fixing roller and a pressure roller that presses against the fixing roller. The fixing member is heated by a heat source, typically but not necessarily internal to the fixing member, and the fixing member and the pressure roller together sandwich the sheet between them to form a fixing nip where the image formed on the sheet is fixed on the sheet with heat and pressure. This method is hereinafter referred to as the heating-roller fixing method.
Recently, various approaches described below have been tried to reduce warm-up time of fixers, thereby reducing energy consumption and waiting time for users. For example, thickness of the fixing roller is reduced or a bubble layer is included in the fixing roller. Alternatively, a fixing member such as an endless belt or film whose heat capacity is smaller than a roller is used. Separately, an electromagnetic induction-heating fixing method has been proposed.
An electromagnetic induction-heating fixer generally includes a so-called excitation coil through which a high-frequency electrical current is passed so as to generate a magnetic flux, and a magnetic core for guiding the magnetic flux to a roller-shaped or belt-shaped heat generator efficiently. A fixing nip can be formed by the heat generator and a pressure roller that presses against the heat generator either directly or indirectly via a fixing member. When the pressure roller presses against the heat generator directly, the heat generator serves as the fixing member.
The magnetic flux causes an eddy current in the heat generator, and thus the heat generator is heated inductively. In this configuration, the heat generator can be promptly heated because the heat generator itself can generate heat, eliminating preheating that is required in the heating-roller fixing method. Thus, the electromagnetic induction-heating fixing method is advantageous in that both warm-up time and energy consumption can be reduced.
However, the electromagnetic induction-heating fixing method still has a problem described below in detail.
Generally, the image forming apparatus can accommodate a variety of different sheet sizes. When sheets whose length in an axial direction of the heat generator (hereinafter simply “width of the sheet”) is relatively small pass through the fixing nip continuously, lateral end portions of the heat generator (or the fixing member including such a heat generator) where the sheets do not pass (hereinafter also “non-sheet area”) tend to overheat.
This is because, although the heat capacity of a typical heat generator is relatively small, heat is drawn from a center portion in the axial direction of the heat generator where the sheet passes (hereinafter “center portion” or “sheet area”) by the sheets whereas heat from the lateral end portions where the sheets do not pass is not lost, inviting overheating in the end portions of the heat generator (hereinafter also simply “peripheral overheating”). Such overheating can degrade or even damage the heat generator.
This peripheral overheating and its resultant uneven temperature distribution have consequences for image quality. Thus, when a sheet whose width is larger than that of the small sheets described above passes through the fixing nip after the small sheets have passed the fixing nip continuously for some time, the level of gloss in a resulting image will be different between a portion fixed by the center portion and a portion fixed by the lateral end portions of the heat generator. If such overheating in the end portions of the heat generator is significant, toner in the resulting image will be partly absent from portions that pass the overheated end portions of the heat generator, which is a phenomenon called hot offset. Hot offset occurs because, when toner is heated excessively, cohesion among toner particles is lower than adhesion between the toner particles and the fixing member, thereby, causing toner layers to separate.
In view of the foregoing, one known technique uses sub-induction coils or demagnetization coils to counteract the magnetic flux generated by a main induction coil or excitation coil. The demagnetization coils are respectively provided in end portions of the heat generator except a sheet area to be covered by a sheet whose width is smallest (hereinafter “smallest sheet”) among multiple different sheet sizes that the image forming apparatus can accommodate. Then, during a fixing operation, the amount of heat generated in the non-sheet areas is reduced from that generated in the sheet area, thus restricting overheating of the heating generator.
In another known method, activation of the demagnetization coils is adjusted according to sheet size because heating might be insufficient if the demagnetization coils are constantly on.
However, in these methods, when small sheets are continuously passed through the fixing nip, even when the demagnetization coils restrict the excessive temperature rise of the heating generator, the temperature of the non-sheet area is higher than that of the sheet area. Therefore, when a relatively large sheet is passed through the fixing nip immediately after small sheets are continuously passed through the fixing nip, the gloss level can be uneven between the center portion and the lateral end portions of the sheet.
In view of the foregoing, there is a need to equalize temperature distribution in the sheet width direction or axial direction of the heat generator after small sheets are continuously passed through the fixer, which the known methods fail to do.